The Internet has widely been used in recent years and users can access various types of information on sites operated all over the world and can obtain such information. Use of devices adapted to broadband access such as asymmetric digital subscriber line (ADSL) and fiber to the home (FTTH) has also rapidly spread accordingly.
IEEE Std 802.3ah™-2004 (NPD 1) discloses one scheme for a passive optical network (PON). The passive optical network realizes medium-sharing-type communication in which a plurality of optical network units (ONUs) share an optical communication line and transmit data with an optical line terminal (OLT). Namely, NPD 1 defines Ethernet® PON (EPON), under which all information including user information passing through a PON and control information for administering and operating a PON is communicated in a form of an Ethernet® frame as well as an access control protocol (multi-point control protocol (MPCP)) and an operations, administration and maintenance (OAM) protocol for EPON. By exchanging MPCP frames between an optical line terminal and an optical network unit, joining, leaving, upstream multiple access control, or the like of an optical network unit is carried out. NPD 1 describes a method of registering a new optical network unit, a report showing a request for allocation of a band, and a gate indicating a transmission instruction based on an MPCP message.
A gigabit Ethernet® passive optical network (GE-PON) is an EPON realizing a communication rate of 1 gigabit/second. A next-generation technique for the GE-PON includes 10 G-EPON standardized as IEEE802.3av™-2009. 10 G-EPON is an EPON in which a communication rate is adapted to 10 gigabits/second. In 10 G-EPON as well, an access control protocol is premised on the MPCP.
A light-emitting element generally used as a light emitter for transmission in an optical communication device such as an ONU and an optical line terminal has optical characteristics as follows. Namely, light emission efficiency representing relation between an injected current and output light has strong temperature dependency. Aging of a light-emitting element also deteriorates characteristics of light emission efficiency. Therefore, it is important to control the light-emitting element in order to adapt to an environmental temperature in a wide range and aging and to obtain desired optical output power, that is, DC characteristics, and a desired extinction ratio, that is, AC characteristics.
For an ONU adapted to 10 G-EPON, for example, a direct modulation scheme in which a bias current and a modulation current supplied to a light-emitting element are directly controlled has been adopted. Here, the modulation current is a current having magnitude in accordance with a logical value of data to be transmitted.
In this direct modulation scheme, for a bias current, for example, such a method that a light-receiving element for monitoring receives backward light in proportion to forward light from a light-emitting element and feedback of a quantity of received light is given to a bias current supply circuit has been adopted.
For a modulation current, a method of feedforward control with the use of a look-up table showing correspondence between an ambient temperature of a device and a control value for a modulation current is possible.
Alternatively, as a method of controlling a modulation current, in an optical communication device of a relatively low speed such as 1 Gbps and 2.5 Gbps, a method of feedback control to a modulation current supply circuit by sensing amplitude of a signal output from a light-receiving element for monitoring also for a modulation current is possible (for example, see PTD 1 (WO2007/103803)).
A scheme below is possible for an optical communication device in which optical signals are successively transmitted. Namely, separately from a main signal, for example, with a period of approximately 100 ms, a low-speed and low pilot current having amplitude corresponding to several % of amplitude of the main signal is superimposed on a supply current for a light-emitting element. An amount of change in signal having the period of approximately 100 ms based on backward light is monitored. Then, light emission efficiency is calculated based on a result of monitoring, and feedback of the result of calculation is given to a modulation current supply circuit (see, for example, PTD 2 (WO98/43330)).